An AC servomotor employing the outer circumference of a stator core formed by laminating magnetic steel plates as part of a motor housing has hithertofore been known. This AC servomotor is a synchronous motor having a rotor provided with field poles formed by permanent magnets and disposed inside a stator or an induction motor having a rotor provided with coils. When such a motor is used as a servomotor, particularly high mechanical accuracy is required, for example, in the concentricity of the rotor with the stator and the concentricity of a motor output shaft attached to the rotor with and the squareness of the same to the motor mount of a driven body, such as a machine tool.
Referring to FIG. 1, a conventional AC servomotor of this kind comprises a stator core 1 having laminated thin steel plates 2, stator coils 3 wound on the stator core 1, a pair of brackets 4 and 5 firmly fixed to the opposite ends of the stator core 1, respectively, with bolts or the like, and a rotor 8 supported rotatably in bearing members 6 and 7 on the brackets 4 and 5. The AC servomotor of such a construction requires a high mechanical accuracy in the concentricity of the rotor 8 with the stator 1 to enable the rotor 8 to rotate smoothly. A high mechanical accuracy is required also in the concentricity of an output shaft 8a fixed to the rotor 8 with and the squareness of the same to a motor mount 9a of a machine tool 9 or the like.
However, since the machining of the stator core 1 and the brackets 4 and 5 of the conventional AC servomotor of this kind is completed before they are joined firmly together, the final accuracy of the above-mentioned concentricity is deteriorated greatly by the accumulation of errors in respective elements and parts when the stator core 1 and the brackets 4 and 5 are assembled. Furthermore, since the stator core 1 and the brackets 4 and 5 are machined before they are assembled, the joining parts of the stator core 1 and the brackets 4 and 5 must be finished with a high mechanical accuracy, which increases precision machining work. Still further, the stator core 1 itself needs to be extremely firm and rigid and separate thick end plates 10 and 11 need to be provided at the opposite ends, respectively, of the laminated thin steel plates of the stator core 1, as illustrated in FIG. 1, in order to finish the joining surfaces of the stator core 1 to contact the brackets with a high accuracy. Such a construction requires annular mounting surface-to be joined to the brackets to be formed in the outer surfaces of the end plates 10 and 11, respectively, which disadvantageously increases the manufacturing cost of the stator and increases work for manufacturing the same.